PROFILE DIES | MACHINERY
Simulation software ensures the die is cast
Simulation software is a critical factor in helping to design and improve the performance of profile dies, for a wide range of applications. Lou Reade reports
The modern design process relies heavily on computer simulation – and extrusion dies are no exception. The complex equations needed to model flow behaviour inside a die are adequately captured by a series of increasingly sophisticated modelling programs, which help to ensure that the final product – such as a profile – hits close toler- ance limits.
Mahesh Gupta, managing director of US-based
simulation software company Plastic Flow, told delegates at this year’s Antec – which was held alongside the NPE show earlier this month – about a method to model cooling shrinkage on extrudate distortion.
He said this was achieved thanks to improved post-die analysis in his company’s PolyXtrue extrusion die design software. He used the enhanced software to predict
extrudate distortion for three different profile dies. The analysis included the effect of both non-uni- form exit velocity and cooling shrinkage on distortion. As an extruded polymer exits a die, the shape of
the profile can change significantly. There are two elements to this distortion: shape change, due to non-uniform velocity at the die exit; and a reduc- tion in cross-section, due to cooling shrinkage at room temperature. After the polymer leaves the die, the thickness and length of the profile will tend to increase at locations with larger velocity and decrease at lower velocity locations. Such non-uniform velocity can cause significant distortion in profile shape. In earlier work, Gupta only included the effect
of non-uniform exit velocity to predict the distortion – but now the effect of cooling shrink- age has been included, to predict the shrinkage of many different extrudate profiles, he said. An amorphous polymer (ABS) was used for all profiles. Effect of temperature on
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the mechanical properties (elastic modulus, Poisson’s ratio, and thermal expansion coefficient) of ABS was captured in the simulation. Flow inside the die is simulated using a number
of equations. The existing commercial version of PolyXtrue solves only five of them – which are flow and heat equations – but the enhanced version also solves five thermo-mechanical equations. The extended version was used in the study – and will be included in the next commercial release. This enhanced version of PolyXtrue includes
extrudate shrinkage analysis capabilities, and was used to predict the shrinkage for three different dies – a circular channel, a square channel, and a profile die. Results for the circular and the square channels
were very similar, so Gupta only presented results for the square channel during Antec. However, it was important to simulate the extrudate shrinkage for a circular channel in order to validate the newly developed extrudate shrinkage code in the software. For the circular channel, the predicted shrinkage matched the corresponding theoretical values.
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Main image: Greiner’s Flow Control
technology adjusts die temperature to balance flow, which can cut material costs
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